I sold a used lens in mint condition, unmarked and fully functioning. I did the usual check and ensured no visible dust or fungus, went over every feature etc.

Several weeks later the recipient is threatening legal action unless I refund the entire money. They complained dust was now visible in the lens.

They said the dust had been there for years and that a shop had verified this. My response was I believe fair and I can not see how you determine when dust entered. I have offered to get the Canon service Centre to look at it if necessarily at my cost.

The purchaser is aggressive and not prepared to consider anything but a refund or legal threats.

I think you'd be better served directing this question to a lawyer rather than a bunch of amateur photographers.

As it is a public holiday here I have no chance of contacting a lawyer. Perhaps people who can contribute ideas may point me in the right direction as to the best course of action.

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